The present invention relates to a lamp for generating ultraviolet radiation used for purification of various types of gases and liquids and for various types of sterilization processes.
In an ultraviolet radiation lamp, for example, a germicidal lamp, a pair of electrodes are provided at two ends of a discharge bulb, and a rare gas and mercury are sealed in the bulb. The lighting principle of the ultraviolet radiation lamp is completely the same as that of a known fluorescent lamp. The ultraviolet radiation lamp is different from the fluoroescent lamp in that it does not use a phosphor film and that its bulb is made of a glass having a good far ultraviolet ray transmittance of fused quartz. In such an ultraviolet radiation lamp, excited mercury atoms emit far ultraviolet rays. Therefore, the ultraviolet radiation lamp is used in a wide range of applications such as sterilization at waterworks and sewage-treatment plants, sterilization of various types of gases, and production, processing, and treatment of products.
However, in a conventional ultraviolet radiation lamp, an arc loading is 1 Watt per centimeter or less, and a total power input per lamp is as low as about 100 W. As a result, the output is comparatively low.
The germicidal ability of a low power ultraviolet radiation lamp is naturally low. When such a low power ultraviolet radiation lamp uses in a large system such as a purification plant, a large number of lamps are required, and the number of additional components is accordingly increased. Therefore, a demand has recently arisen for development of a high power ultraviolet radiation lamp.
A high power ultraviolet radiation lamp as described in Japanese Patent Disclosure (Kokai) No. 56-160755 is known. This Disclosure discloses an ultaviolet radiation lamp using a light-emitting tube made of ozone-less fused quartz. A pair of electrodes comprising a cathode and an anode are provided at two end of the light-emitting tube. A rare gas and mercury are sealed in the light-emitting tube. The arc length of the light-emitting tube is 300 mm, the lamp current is 4 A, and the power consumption is about 200 W.
However, recently, an ultraviolet radiation lamp having a higher output than that described in the above Disclosure is demanded. Therefore, a high power ultraviolet radiation lamp which can be turned on with an arc length of 1,000 mm or more and a lamp input density (input power per unit arc length) of 3 to 10 W/cm must be developed.
In such a high power ultraviolet radiation lamp, in order to obtain far ultraviolet rays having a wavelength of 254 nm, i.e., germicidal rays, optimal control of the mercury vapor pressure is important. More specifically, in a high power ultraviolet radiation lamp having an arc length of 1,000 mm or more and a lamp input density of 3 to 10 W/cm, the input density (lamp input per unit arc length) during ON time is about 10 times that of a conventional 100-W ultraviolet radiation lamp. Therefore, when the high power ultraviolet radiation lamp is lighting under a natural cooling (air cooling) condition, the tube wall temperature of the light-emitting tube reaches as high as 150.degree. to 250.degree. C. When mercury is sealed in such a light-emitting tube in the same manner as in a conventional fluorescent lamp, an optimum mercury vapor pressure cannot be obtained easily.
According to Japanese Patent Disclosure (Kokai) No. 60-143554, a light-emitting tube is housed in a container. The light-emitting tube is forcibly cooled by flowing water in the container. As a result, the tube wall temperature of the light-emitting tube can be maintained properly to provide an optimum mercury vapor pressure. However, in such a lamp, the flow rate and temperature of water must be strictly controlled. In addition, a means for preventing water from leaking from the container or a means for insulating the electrically conductive portion becomes complex and large. Therefore, such a lamp is not practical.
U.S. Pat. No. 4,349,765 discloses an appendix-like tubular portion for reserving excessive mercury liquid. The mercury vapor pressure is adjusted by controlling the temperature of the tubular portion. However, in this case, the mercury vapor pressure cannot be kept constant depending on the gas temperature in the vicinity of the appendix-like tubular portion or the flowing state of the gas. Therefore, the lamp characteristics can easily fluctuate.